Watson Smith

Artificial Substantive Dyestuffs.—You may remember that in the last lecture we divided the colouring matters as follows: I. Substantive colours, fixing themselves directly on animal fibres without a mordant, only a few of them doing this, however, on vegetable fibres, like cotton. We sub-divided them further as—(a) those occurring in nature, and (b) those prepared artificially, and chiefly, but not entirely, the coal-tar colouring matters. II. Adjective colours, fixing themselves only in conjunction with a mordant or mordants on animal or vegetable fibres, and including all the polygenetic colours. III. Mineral or pigment colours. I described experiments to illustrate what we mean by monogenetic and polygenetic colours, and indicating that the monogenetic colours are mainly included in the group of substantive colours, whilst the polygenetic colours are mainly included in the adjective colours. But I described also an illustration of Group III., the mineral or pigment colours, by which we may argue that chromate of lead is a polygenetic mineral colour, for, according to the treatment, we were able to obtain either chrome yellow (neutral lead chromate) or chrome orange (basic lead chromate). I also said there was a kind of borderland whichever mode of classification be adopted. Thus, for example, there are colours that are fixed on the fibre either directly like indigo, and so are substantive, or they may be, and generally are, applied with a mordant like the adjective and polygenetic colours; examples of these are Coerulein, Alizarin Blue, and a few more. We have now before us a vast territory, namely, that of the b group of substantive colours, or, the largest proportion, indeed almost all of those prepared from coal-tar sources; Alizarin, also prepared from coal-tar, belongs to the adjective colours. With regard to the source of these coal-tar colours, the word "coal-tar," I was going to say, speaks volumes, for the destructive and dry distillation of coal in gas retorts at the highest temperatures to yield illuminating gas, also yields us tar. But, coal distilled at lower temperatures, as well as shale, as in Scotland, will yield tar, but tar of another kind, from which colour-generating substances cannot be obtained practically, but instead, paraffin oil and paraffin wax for making candles, etc. Coal-tar contains a very large number of different substances, but only a few of them can be extracted profitably for colour-making. All the useful sources of colours and dyes from coal-tar are simply compounds of carbon and hydrogen—hydrocarbons, as they are called, with the exception of one, namely, phenol, or carbolic acid. I am not speaking here of those coal-tar constituents useful for making dyes, but of those actually extracted from coal-tar for that purpose, i.e. extracted to profit. For example, aniline is contained in coal-tar, but if we depended on the aniline contained ready made in coal-tar for our aniline dyes, the prices of these dyes would place them beyond our reach, would place them amongst diamonds and precious stones in rarity and cost, so difficult is it to extract the small quantity of aniline from coal-tar. The valuable constituents actually extracted are then these: benzene, toluene, xylene, naphthalene, anthracene, and phenol or carbolic acid. One ton of Lancashire coal, when distilled in gas retorts, yields about 12 gallons of coal-tar. Let us now learn what those 12 gallons of tar will give us in the shape of hydrocarbons and carbolic acid, mentioned as extracted profitably from tar. This is shown very clearly in the following table (Table A).

The 12 gallons of tar yield 1-1/10 lb. of benzene, 9/10 lb. of toluene, 1½ lb. of carbolic acid, between 1/10 and 2/10 lb. of xylene, 6½ lb. of naphthalene, and ½ lb. of anthracene, whilst the quantity of pitch left behind is 69½ lb. But our table shows us more; it indicates to us what the steps are from each raw material to each colouring matter, as well as showing us each colouring matter. We see here that our benzene yields us an equal weight of aniline, and the toluene (9/10 lb.) about 3/4 lb. of toluidine, the mixture giving, on oxidation, between ½ and 3/4 lb of Magenta. From carbolic acid are obtained both Aurin and picric acid, and here is the actual quantity of Aurin obtainable (1-1/4 lb.). From naphthalene, either naphthylamine (a body like aniline) or naphthol (resembling phenol) may be prepared. The amounts obtainable you see in the table. There are two varieties of naphthol, called alpha- and beta-naphthol, but only one phenol, namely, carbolic acid. Naphthol Yellow is of course a naphthol colour, whilst Vermilline Scarlet is a dye containing both naphthylamine and naphthol. You see the quantities of these dyes, namely 7 lb. of Scarlet and 9½ lb. of the Naphthol Yellow. The amount of pure anthracene obtained is ½ lb. This pure anthracene exhibits the phenomenon of fluorescence, that is, it not only looks white, but when the light falls on it, it seems to reflect a delicate violet or blue light. Our table shows us that from the 12 gallons of tar from 1 ton of coal we may gain 2-1/4 lb. of 20 per cent. Alizarin paste. Chemically pure Alizarin crystallises in bright-red needles; it is the colouring principle of madder, and also of Alizarin paste. But the most wonderful thing about substantive coal-tar colours is their immense tinctorial power, i.e. the very little quantity of each required compared with the immense superficies of cloth it will dye to a full shade.

TABLE A.[1]

Twelve Gallons of Gas-Tar (average of Manchester and Salford Tar) yield:—

Benzene.	Toluene.	Phenol.	Solvent Naphtha for India rubber, containing the three Xylenes.	Heavy Naphtha.	Naphthalene.	Creosote.	Heavy Oil	Anthracene.	Pitch.
1·10 lb.= 1·10 lb. of Aniline	0·90 lb.= 0·77 lb. of Toluidine.	1·5 lb. = 1·2 lb. of Aurin.	2·44 lb., yielding 0·12 lb. of Xylene = 0·07 lb. of Xylidine.	2·40 lb.	6·30 lb. = 5·25 lb. of α-Naphthylamine= 7·11 lb. of Vermilline Scarlet RRR; or 4·75 lb. of α- or β- Naphthol = 9·50 lb. of Naphthol Yellow.	17 lb.	14 lb.	0·46 lb. = 2·25 lb. of Alizarin (20%).	69·6 lb.
\_____________	___/
= 0·623 lb of	Magenta.
or 1·10 lb. of Aniline yields 1·23 lb. of Methyl Violet.

[1] This table was compiled by Mr. Ivan Levinstein, of Manchester.

The next table (see Table B) shows you the dyeing power of the colouring matters derived from 1 ton of Lancashire coal, which will astonish any thoughtful mind, for the Magenta will dye 500 yards of flannel, the Aurin 120 yards, the Vermilline Scarlet 2560 yards, and the Alizarin 255 yards (Turkey-red cotton cloth).

The next table (Table C) shows the latent dyeing power resident, so to speak, in 1 lb. of coal.

By a very simple experiment a little of a very fine violet dye can be made from mere traces of the materials. One of the raw materials for preparing this violet dye is a substance with a long name, which itself was prepared from aniline. This substance is tetramethyldiamidobenzophenone, and a little bit of it is placed in a small glass test-tube, just moistened with a couple of drops of another aniline derivative called dimethylaniline, and then two drops of a fuming liquid, trichloride of phosphorus, added. On simply warming this mixture, the violet dyestuff is produced in about a minute. Two drops of the mixture will colour a large cylinder of water a beautiful violet. The remainder (perhaps two drops more) will dye a skein of silk a bright full shade of violet. Here, then, is a magnificent example of enormous tinctorial power. I must now draw the rein, or I shall simply transport you through a perfect wonderland of magic, bright colours and apparent chemical conjuring, without, however, an adequate return of solid instruction that you can carry usefully with you into every-day life and practice.

TABLE B.[1]

Dyeing Powers of Colours from 1 Ton of Lancashire Coal.

TABLE C.

Dyeing Powers of Colours from 1 Lb. of Lancashire Coal.

Before we go another step, I must ask and answer, therefore, a few questions. Can we not get some little insight into the structure and general mode of developing the leading coal-tar colours which serve as types of whole series? I will try what can be done with the little knowledge of chemistry we have so far accumulated. In our earlier lectures we have learnt that water is a compound of hydrogen and oxygen, and in its compound atom or molecule we have two atoms of hydrogen combined with one of oxygen, symbolised as H₂O. We also learnt that ammonia, or spirits of hartshorn, is a compound of hydrogen with nitrogen, three atoms of hydrogen being combined with one of nitrogen, thus, NH₃. An example of a hydrocarbon or compound of carbon and hydrogen, is marsh gas (methane) or firedamp, CH₄. Nitric acid, or aqua fortis, is a compound of nitrogen, oxygen, and hydrogen, one atom of the first to three of the second and one of the third—NO₃H. But this nitric acid question forces me on to a further statement, namely, we have in this formula or symbol, NO₃H, a twofold idea—first, that of the compound as a whole, an acid; and secondly, that it is formed from a substance without acid properties by the addition of water, H₂O, or, if we like, HOH. This substance contains the root or radical of the nitric acid, and is NO₂, which has the power of replacing one of the hydrogen atoms, or H, of water, and so we get, instead of HOH, NO₂OH, which is nitric acid. This is chemical replacement, and on such replacement depends our powers of building up not only colours, but many other useful and ornamental chemical structures. You have all heard the old-fashioned statement that "Nature abhors a vacuum." We had a very practical example of this when in our first lecture on water I brought an electric spark in contact with a mixture of free oxygen and hydrogen in a glass bulb. These gases at once united, three volumes of them condensing to two volumes, and these again to a minute particle of liquid water. A vacuum was left in that delicate glass bulb whilst the pressure of the atmosphere was crushing with a force of 15 lb. on the square inch on the outside of the bulb, and thus a violent crash was the result of Nature's abhorrence. There is such a kind of thing, though, and of a more subtle sort, which we might term a chemical vacuum, and it is the result of what we call chemical valency, which again might be defined as the specific chemical appetite of each substance.

Let us now take the case of the production of an aniline colour, and let us try to discover what aniline is, and how formed. I pointed to benzene or benzol in the table as a hydrocarbon, C₆H₆, which forms a principal colour-producing constituent of coal-tar. If you desire to produce chemical appetite in benzene, you must rob it of some of its hydrogen. Thus C₆H₅ is a group that would exist only for a moment, since it has a great appetite for H, and we may say this appetite would go the length of at once absorbing either one atom of H (hydrogen) or of some similar substance or group having a similar appetite. Suppose, now, I place some benzene, C₆H₆, in a flask, and add some nitric acid, which, as we said, is NO₂OH. On warming the mixture we may say a tendency springs up in that OH of the nitric acid to effect union with an H of the C₆H₆ (benzene) to form HOH (water), when an appetite is at once left to the remainder, C₆H₅—on the one hand, and the NO₂—on the other, satisfied by immediate union of these residues to form a substance C₆H₆NO₂, nitro-benzene or "essence of mirbane," smelling like bitter almonds. This is the first step in the formation of aniline.

I think I have told you that if we treat zinc scraps with water and vitriol, or water with potassium, we can rob that water of its oxygen and set free the hydrogen. It is, however, a singular fact that if we liberate a quantity of fresh hydrogen amongst our nitrobenzene C₆H₅NO₂, that hydrogen tends to combine, or evinces an ungovernable appetite for the O₂ of that NO₂ group, the tendency being again to form water H₂O. This, of course, leaves the residual C₆H₅N: group with an appetite, and only the excess of hydrogen present to satisfy it. Accordingly hydrogen is taken up, and we get C₆H₅NH₂ formed, which is aniline. I told you that ammonia is NH₃, and now in aniline we find an ammonia derivative, one atom of hydrogen (H) being replaced by the group C₆H₅. I will now describe the method of preparation of a small quantity of aniline, in order to illustrate what I have tried to explain in theory. Benzene from coal-tar is warmed with nitric acid in a flask. A strong action sets in, and on adding water, the nitrobenzene settles down as a heavy oil, and the acid water can be decanted off. After washing by decantation with water once or twice, and shaking with some powdered marble to neutralise excess of acid, the nitrobenzene is brought into contact with fresh hydrogen gas by placing amongst it, instead of zinc, some tin, and instead of vitriol, some hydrochloric acid (spirits of salt). To show you that aniline is formed, I will now produce a violet colour with it, which only aniline will give. This violet colour is produced by adding a very small quantity of the aniline, together with some bleaching powder, to a mixture of chalk and water, the chalk being added for the purpose of destroying acidity. This aniline, C₆H₅NH₂, is a base, and forms the foundation of all the so-called basic aniline colours. If I have made myself clear so far, I shall be contented. It only remains to be said that for making Magenta, pure aniline will not do, what is used being a mixture of aniline, with an aniline a step higher, prepared from toluene. If I were to give you the formula of Magenta you would be astonished at its complexity and size, but I think now you will see that it is really built up of aniline derivatives. Methyl Violet is a colour we have already referred to, and its chemical structure is still more complex, but it also is built up of aniline materials, and so is a basic aniline colour. Now it is possible for the colour-maker to prepare a very fine green dye from this beautiful violet (Methyl Violet). In fact he may convert the violet into the green colour by heating the first under pressure with a gas called methyl chloride (CH₃Cl). Methyl Violet is constructed of aniline or substituted aniline groups; the addition of CH₃Cl, then, gives us the Methyl Green. But one of the misfortunes of Methyl Green is that if the fabric dyed with it be boiled with water, at that temperature (212° F.) the colour is decomposed and injured, for some of the methyl chloride in the compound is driven off. In fact, by stronger heating we may drive off all the methyl chloride and get the original Methyl Violet back again.

But we have coal-tar colours which are not basic, but rather of the nature of acid,—a better term would be phenolic, or of the nature of phenol or carbolic acid. Let us see what phenol or carbolic acid is. We saw that water may be formulated HOH, and that benzene is C₆H₆. Well, carbolic acid or phenol is a derivative of water, or a derivative of benzene, just as you like, and it is formulated C₆H₅OH. You can easily prove this by dropping carbolic acid or phenol down a red-hot tube filled with iron-borings. The oxygen is taken up by the iron to give oxide of iron, and benzene is obtained, thus: C₆H₅OH gives O and C₆H₆. But there is another hydrocarbon called naphthalene, C₁₀H₈, and this forms not one, but two phenols. As the name of the hydrocarbon is naphthalene, however, we call these compounds naphthols, and one is distinguished as alpha- the other as beta-naphthol, both of them having the formula C₁₀H₇OH. But now with respect to the colours. If we treat phenol with nitric acid under proper conditions, we get a yellow dye called picric acid, which is trinitro-phenol C₆H₂(NO₂)₃OH; you see this is no aniline dye; it is not a basic colour, for it would saturate, i.e. destroy the basicity of bases. Again, by oxidising phenol with oxalic acid and vitriol, we get a colour dyeing silk orange, namely, Aurin, HO.C[C₆H₄(OH)]₃. This is also an acid or phenolic dye, as a glance at its formula will show you. Its compound atom bristles, so to say, with phenol-residues, as some of the aniline dyes do with aniline residue-groups.

We come now to a peculiar but immensely important group of colours known as the azo-dyes, and these can be basic or acid, or of mixed kind. Just suppose two ammonia groups, NH₃ and NH₃. If we rob those nitrogen atoms of their hydrogen atoms, we should leave two unsatisfied nitrogen atoms, atoms with an exceedingly keen appetite represented in terms of hydrogen atoms as N and N. We might suppose a group, though of two N atoms partially satisfied by partial union with each other, thus—N:N—. Now this group forms the nucleus of the azo-colours, and if we satisfy a nitrogen at one side with an aniline, and at the other with a phenol, or at both ends with anilines, and so on, we get azo-dyes produced. The number of coal-tar colours is thus very great, and the variety also.

Adjective Colours.—As regards the artificial coal-tar adjective dyestuffs, the principal are Alizarin and Purpurin. These are now almost entirely prepared from coal-tar anthracene, and madder and garancine are almost things of the past. Vegetable adjective colours are Brazil wood, containing the dye-generating principle Brasilin, logwood, containing HÆmatein, and santal-wood, camwood, and barwood, containing Santalin. Animal adjective colours are cochineal and lac dye. Then of wood colours we have further: quercitron, Persian berries, fustic and the tannins or tannic acids, comprising extracts, barks, fruits, and gallnuts, with also leaves and twigs, as with sumac. All these colours dye only with mordants, mostly forming with certain metallic oxides or basic salts, brightly-coloured compounds on the tissues to which they are applied.